Through the proposed training in his K01, I will pursue the additional mentorship that I need and the additional research time to enhance my passion for intestinal epithelial biology. During this award period I will have the opportunity to acquire and refine fundamental skills of becoming a developed scientist. Career development will be fostered through twice-weekly meetings with Dr. Rustgi, convening an expert interdisciplinary advisory committee twice annually, and taking advantage of opportunities in career and professional development through UPenn, the NIH and the AGA. These skills include (but are not limited to) manuscript and grant preparation, lab meeting presentations, participation in the seminars and courses described herein, and presentations at international research conferences (DDW, Keystone). Training will also entail course-work in biostatistics (two semesters), short courses in RNA biology and basic bioinformatics, ongoing training in bioethics, weekly participation in the seminar series (where I will meet visiting professors), journal clubs (presentations semi-annually), and twice yearly floo lab meetings through the NIDDK P30 Center for Molecular Studies in Digestive and Liver Diseases and Division of Gastroenterology. The overarching hypothesis of this project is the following: LIN28B plays a critical role in regulating growth and proliferation in the intestinal epithelium via cooperation with c-MYC, with key roles for intestine stem cell (ISC) maintenance. Two potential interrelated mechanisms will be explored: 1) LIN28B binds to target mRNAs associated with cellular growth and cooperates with the transcription factor c-MYC, which is a master regulator of cell growth. and 2) LIN28B affects the maintenance of the ISC by promoting stem cell identity and growth. LIN28B is normally expressed in the intestinal epithelial crypt where cell division occurs. LIN28B represses the maturation of miRNAs such as Let-7, which is restricted to villus epithelium, where post- mitotic differentiated cells are located. The spatial restriction of LIN28B and Let-7 expression in the intestinal epithelium is likely crucial for maintaining sufficient but limited compartments of differentiation and proliferation. C-MYC is also restricted to the crypt epithelium and is required for epithelial proliferation and growth. W have evidence to support a Let-7-independent function of LIN28B entailing intimate cooperation with c-MYC. The Specific Aims of this proposal to test the hypothesis are the followin: 1) We will explore the functional relationship between LIN28B and c-MYC, in vitro. 2) We will assess how LIN28B cooperates with c-MYC through in vivo studies of Lin28b in the intestinal epithelium. 3) We will examine the potential of LIN28B to modulate the intestinal stem cell compartment. LIN28B function may be relevant to ISC homeostasis and epithelial proliferation. Determining how LIN28B functions in the intestinal crypt will likely provide an important link among the cellular mechanisms governing epithelial proliferation, differentiation, and/or mucosal regeneration.